Date Time Parser using YACC shift reduce conflicts - datetime

Question

I have the following YACC parser
%start Start
%token _DTP_LONG // Any number; Max upto 4 Digits.
%token _DTP_SDF // 17 Digit number indicating SDF format of Date Time
%token _DTP_EOS // end of input
%token _DTP_MONTH //Month names e.g Jan,Feb
%token _DTP_AM //Is A.M
%token _DTP_PM //Is P.M
%%
Start : DateTimeShortExpr
| DateTimeLongExpr
| SDFDateTimeExpr EOS
| DateShortExpr EOS
| DateLongExpr EOS
| MonthExpr EOS
;
DateTimeShortExpr : DateShortExpr TimeExpr EOS {;}
| DateShortExpr AMPMTimeExpr EOS {;}
;
DateTimeLongExpr : DateLongExpr TimeExpr EOS {;}
| DateLongExpr AMPMTimeExpr EOS {;}
;
DateShortExpr : Number { rc = vDateTime.SetDate ((Word) $1, 0, 0);
}
| Number Number { rc = vDateTime.SetDate ((Word) $1, (Word) $2, 0); }
| Number Number Number { rc = vDateTime.SetDate ((Word) $1, (Word) $2, (Word) $3); }
;
DateLongExpr : Number AbsMonth { // case : number greater than 31, consider as year
if ($1 > 31) {
rc = vDateTime.SetDateFunc (1, (Word) $2, (Word) $1);
}
// Number is considered as days
else {
rc = vDateTime.SetDateFunc ((Word) $1, (Word) $2, 0);
}
}
| Number AbsMonth Number {rc = vDateTime.SetDateFunc((Word) $1, (Word) $2, (Word) $3);}
;
TimeExpr : Number { rc = vDateTime.SetTime ((Word) $1, 0, 0);}
| Number Number { rc = vDateTime.SetTime ((Word) $1, (Word) $2, 0); }
| Number Number Number { rc = vDateTime.SetTime ((Word) $1, (Word) $2, (Word) $3); }
;
AMPMTimeExpr : TimeExpr _DTP_AM { rc = vDateTime.SetTo24hr(TP_AM) ; }
| TimeExpr _DTP_PM { rc = vDateTime.SetTo24hr(TP_PM) ; }
| _DTP_AM TimeExpr { rc = vDateTime.SetTo24hr(TP_AM) ; }
| _DTP_PM TimeExpr { rc = vDateTime.SetTo24hr(TP_PM) ; }
;
SDFDateTimeExpr : SDFNumber { rc = vDateTime.SetSDF ($1);}
;
MonthExpr : AbsMonth { rc = vDateTime.SetNrmMth ($1);}
| AbsMonth Number { rc = vDateTime.Set ($1,$2);}
;
Number : _DTP_LONG { $$ = $1; }
;
SDFNumber : _DTP_SDF { $$ = $1; }
;
EOS : _DTP_EOS { $$ = $1; }
;
AbsMonth : _DTP_MONTH { $$ = $1; }
;
%%
It is giving three shift reduce conflicts.How can i remove them????

Answer 1

The shift-reduce conflicts are inherent in the "little language" that your grammar describes. Consider the stream of input tokens
_DTP_LONG _DTP_LONG _DTP_LONG EOS
Each _DTP_LONG can be reduced as a Number. But should
Number Number Number
be reduced as a 1-number DateShortExpr followed by a 2-number TimeExpr or as a 2-number DateShortExpr followed by a 1-number TimeShortExpr? The ambiguity is built in.
If possible, redesign your language by adding additional symbols to distinguish dates from times--colons to set off the parts of a time and slashes to set off the parts of a date, for instance.
Update
I don't think that you can use yacc/bison's precedence features here, because the tokens are indistinguishable.
You will have to rely on yacc/bison's default behavior when it encounters a shift/reduce conflict, that is, to shift rather than reduce. Consider this example in your output:
+------------------------- STATE 9 -------------------------+
+ CONFLICTS:
? sft/red (shift & new state 12, rule 11) on _DTP_LONG
+ RULES:
DateShortExpr : Number^ (rule 11)
DateShortExpr : Number^Number
DateShortExpr : Number^Number Number
DateLongExpr : Number^AbsMonth
DateLongExpr : Number^AbsMonth Number
+ ACTIONS AND GOTOS:
_DTP_LONG : shift & new state 12
_DTP_MONTH : shift & new state 13
: reduce by rule 11
Number : goto state 26
AbsMonth : goto state 27
What the parser will do is to shift and apply rule 12, rather than reduce by rule 11 (DateShortExpr : Number). This means the parser will never interpret a single Number as a DateShortExpr; it will always shift.
And a difficulty with relying on the default behavior is that it might change as you make modifications to your grammar.